This application is the National Phase of International Application PCT/SG01/00163 filed 13 Aug. 2001 which designated the U.S. and that International Application was published under PCT Article 21(2) in English.
1. Field of the Invention
This invention relates to an adaptive directional microphone system with high spatial selectivity and noise/interference suppression and, more particularly, to an adaptive directional microphone system capable of suppressing background noise and the undesired signals from the first directions and remaining the desired signal from the second directions, and to a hand-free high spatial selectivity microphone, such as for use with a computer voice input system, a hand-free communication voice input system, or the like.
2. Description of the Related Art
A normal directional microphone system is a microphone system having a directivity pattern. The directivity pattern describes the directional microphone system's sensitivity to sound pressure from different directions. It can provide higher gain at some wider areas in direction normally around the front direction (0°-axis) (in the present invention, referred to as the first directions) and lower gain or even null at some other directions normally around the back direction (referred to as the second directions in the present invention). The purpose of the directional microphone system is to receive sound pressure originating from a desirable sound source, such as speech, and attenuate sound pressure originating from undesirable sound sources, such as noise. The directional microphone system is typically used in noisy environments, such as a vehicle or a public place.
Directional microphones receiving a maximum amount of desired sound from a desired direction and meanwhile rejecting undesired noise at a second or null directions, are generally well known in the prior art. Examples include cardioid-type directional microphones, such as cardioid, hyper-cardioid and super-cardioid directional microphones. However, those microphones are of very broad main beam and very narrow null. In many applications such as computer voice input system or the like, a directional microphone system, which has a narrow main beam with much higher gain than that in the other directions, is required to acquire only the desired sound from one direction and suppress the undesired noise from the any other directions.
One known technique for achieving directionality is through the use of a first-order-gradient (FOG) microphone element which comprises a movable diaphragm with front and back surfaces enclosed within a capsule. The prior arts of directional microphones, such as in U.S. patents U.S. Pat. No. 4,742,548, U.S. Pat. No. 5,121,426, U.S. Pat. No. 5,226,076 and U.S. Pat. No. 5,703,957, etc., only can provide a null with very low gain at certain narrow directions but a beam with high gain at broad directions. In applications for such a microphone, the null of the microphone must be towards the undesired noise source and meanwhile the desired sound source should be positioned at the first directions of the microphone. However, in practice, the arrangement is somewhat cumbersome because sometimes it is difficult to arrange the undesired noise source and desired sound source as above and moreover the noise may not come from a fixed direction. For example, there may be multiple noise sources from different directions or distributed noise source.
A directional microphone system has been previously suggested in the PCT patent application No. PCT/SG00/00080 (not yet published) that uses an omni-directional microphone and a directional microphone with an adaptive filtering circuit to suppress undesired signals from the first directions and retain the desired signal from second directions.
The present invention is to enhance the performance of noise/interference suppression and narrow the range of the main beam for the above invention by a new post-processing scheme.